3D Printed Ceramic to Metal Assemblies for Electric Feedthroughs in Implantable Medical Devices

ABSTRACT

An electrical feedthrough assembly for an implantable medical device includes an outer ferrule of metallic material having an outer surface hermetically sealed to an implantable device housing. There is an inner feedthrough assembly which is hermetically sealed within the ferrule and which has a structure of sintered layers that include: i. an electrical insulator of ceramic insulator material, ii. one or more electrically conductive vias of metallized conductive material embedded within and extending through the electrical insulator, and iii. a transition interface region around each of the conductive vias comprising a gradient mixture of the ceramic insulator material and the metallized conductive material forming a gradual transition and a mechanical bond between the electrical insulator and the conductive via.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/158,646 filed May 19, 2016, now U.S. Pat. No. 10,376,703, whichclaims priority from U.S. Provisional Patent Application No. 62/164,017filed May 20, 2015, which is incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to implantable medical devices, and morespecifically to a hermetically sealed electrical feedthrough for suchdevices.

BACKGROUND ART

A normal ear transmits sounds as shown in FIG. 1 through the outer ear101 to the tympanic membrane 102, which moves the bones of the middleear 103 that vibrate the oval window and round window openings of thecochlea 104. The cochlea 104 is a long narrow duct wound spirally aboutits axis for approximately two and a half turns. It includes an upperchannel known as the scala vestibuli and a lower channel known as thescala tympani, which are connected by a central cochlear duct. Thecochlea 104 forms an upright spiraling cone with a center called themodiolar where the spiral ganglion cells of the acoustic nerve 113reside. In response to received sounds transmitted by the middle ear103, the fluid-filled cochlea 104 functions as a transducer to generateelectric pulses which are transmitted to the cochlear nerve 113, andultimately to the brain.

Hearing is impaired when there are problems in the ability to transduceexternal sounds into meaningful action potentials along the neuralsubstrate of the cochlea 104. To improve impaired hearing, auditoryprostheses have been developed. When the impairment is associated withthe cochlea 104, a cochlear implant with an implanted electrode canelectrically stimulate auditory nerve tissue with small currentsdelivered by multiple electrode contacts distributed along theelectrode.

FIG. 1 also shows some components of a typical cochlear implant systemwhere an external microphone provides an audio signal input to anexternal signal processor 111 in which various signal processing schemescan be implemented. The processed signal is then converted into adigital data format for transmission by external transmitter coil 107into the implant device 108. Besides receiving the processed audioinformation, the implant device 108 also performs additional signalprocessing such as error correction, pulse formation, etc., and producesan electrical stimulation pattern (based on the extracted audioinformation) that is sent through an electrode lead 109 to an implantedelectrode array 110. The electrode array 110 includes multiple electrodecontacts 112 on its outer surface that provide selective stimulation ofthe adjacent neural tissues within the cochlea 104.

The implant device 108 connects to the electrode lead 109 at an electricfeedthrough on the outer surface of the implant device 108. The electricfeedthrough includes an electrical insulator which contains one or moreelectrically conductive connectors. The feedthrough is surrounded by anouter ferrule that is hermetically sealed to the housing of the implantdevice 108. In addition, the electrical insulator and the conductiveconnectors also must form a hermetical seal within the ferrule toprevent fluids and moisture from entering into the interior of theimplant device 108.

To form the required hermetic seal of the electrical feedthrough, thecritical issue is the interface between metallic feedthrough ferrule andthe ceramic material of the implant device. Ceramics and metals havedifferent material properties which make it difficult to seal thedissimilar materials. Ceramics exhibit an ionic bonding while metalshave a metallic bonding that complicates wetting of the ceramic with themetal, which results in poor adhesion of the metal to a ceramic surface.The ceramic and the metal materials also exhibit different thermalexpansion coefficients which leads to thermal stress problems.

Various techniques are available to hermetically seal glass/ceramics andmetals. For a hermetic glass-to-metal seal, the molten glass must becapable of wetting the metal so that a tight bond is formed. The glassand the metal are strongly bond together when the oxide layer at themetal surface chemically interacts with the glass. In addition, thethermal expansion coefficients of the glass and the metal need to matchin order to achieve a stable seal when the assembly cools down. Furtherhermetic connections can be provided by glass-ceramic-to-metal seals.Glass-ceramics are polycrystalline ceramic materials formed bycontrolled crystallization in order to adapt the thermal expansioncoefficient of the glass-ceramics to the one of metal. Glass-ceramic-tometal seals are in use for electrical feedthroughs in vacuum applicationor for hermetic and insulating sealants in solid oxide fuel cells.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to an electricalfeedthrough assembly for an implantable medical device. An outer ferruleof metallic material has an outer surface hermetically sealed to animplantable device housing. There is an inner feedthrough assembly whichis hermetically sealed within the ferrule and which has a structure ofsintered layers that include: i. an electrical insulator of ceramicinsulator material, ii. one or more electrically conductive vias ofmetallized conductive material embedded within and extending through theelectrical insulator, and iii. a transition interface region around eachof the conductive vias comprising a gradient mixture of the ceramicinsulator material and the metallized conductive material forming agradual transition and a mechanical bond between the electricalinsulator and the conductive via.

In specific embodiments, the electrical insulator may include aninsulator outer surface of metallic material adapted for hermeticsealing to the outer ferrule; for example, by sintering or brazing.There may also be one or more electrical circuit components embeddedwithin the electrical insulator. The sintered layers may be based onmaterial deposits made by a 3D printer. For example, the ceramicinsulator material may be made of sintered ceramic powder particles.

Embodiments of the present invention also include a cochlear implantdevice having an electrical feedthrough assembly as described herein.

Embodiments of the present invention also include a method of producingan electrical feedthrough assembly for an implantable medical device. Anouter ferrule is provided made of metallic material having an outersurface hermetically sealed to an implantable device housing. An innerfeedthrough assembly is produced including: i. providing ceramicinsulator material to form an electrical insulator, ii. providing one ormore electrically conductive vias of metallized conductive materialembedded within and extending through the electrical insulator, and iii.sintering the ceramic insulator material and the one or moreelectrically conductive layers to form a hermetic boundary that includesa transition interface region around each of the conductive viascomprising a gradient mixture of the ceramic insulator material and themetallized conductive material forming a gradual transition and amechanical bond between the electrical insulator and the conductive via.The inner feedthrough assembly is located within the outer ferrule, anda hermetic seal is formed between the inner feedthrough assembly and theouter ferrule.

In specific embodiments, the hermetic seal between the inner feedthroughassembly and the outer ferrule may be formed by sintering or brazing.Producing the inner feedthrough assembly also may include embedding oneor more electrical circuit components within the electrical insulator.The ceramic insulator material and the one or more electricallyconductive vias may be produced by a 3D printing process. The ceramicinsulator material may include ceramic powder particles.

Embodiments of the present invention also include a cochlear implantdevice having an electrical feedthrough assembly produced by a method asdescribed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the anatomy of the human ear with a cochlear implantsystem.

FIG. 2 shows a cross-sectional view of an electrical feedthroughassembly according to an embodiment of the present invention.

FIG. 3 shows various steps in a method of producing an electricalfeedthrough assembly according to an embodiment of the presentinvention.

FIG. 4 shows a cross-sectional view of an electrical feedthroughassembly according to another embodiment.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Embodiments of the present invention are directed to an electricalfeedthrough assembly for an implantable medical device that is producedby a 3D printing process. The electrical insulator structure andelectrically conductive vias embedded within the insulator are printedin a single process, which reduces the number of joining steps. Theinsulator and the conductive vias are sintered together in combinationto provide firmly bonded interlocking between the materials that forms ahermetic seal. And as a result of the printing and sintering processes,the insulating and conducting materials interfuse where they meet acrossa gradient interface without a clear separation boundary.

3D printing presents an opportunity to create 3-dimensional structuresin the bulk material of the electrical insulator that facilitateattachment of conductive members in which it is easy to create differentpatterns, sizes and geometries of electric contact areas on the opposingsides of the feedthrough component. 3D printing also allows a gradualchange in the composition of the electrically conductive vias fromoxidation susceptible materials (e.g. niobium, tantalum, etc.) withinthe bulk insulator material, to less reactive materials (e.g. platinum)at the outer surfaces.

FIG. 2 shows a cross-sectional view of an electrical feedthroughassembly 200 according to an embodiment of the present invention, andFIG. 3 shows various steps in a method of producing such an electricalfeedthrough assembly 200. The electrical feedthrough assembly 200provides an outer ferrule 201, step 301, that is made of metallicmaterial. The outer surface of the outer ferrule 201 is hermeticallysealed to the housing of an implantable device (not shown).

An inner feedthrough assembly is developed by a 3D printing process thatprovides one or more layers of ceramic insulator material, step 302,that form an electrical insulator 202, and one or more electricallyconductive vias 203, step 303, which are made of metallized conductivematerial that are embedded within and extend through the electricalinsulator 202. The ceramic insulator material of the electricalinsulator 202 and the metallized conductive material of the one or moreconductive vias 203 are formed by deposits of the material by a 3Dprinter in a series of layers in a single 3D printing process. Thesource materials for the electric insulator 202 may typically be apowder bed made of ceramic powder particles provided to a 3D printer asa powder in a fluid environment. For example, ceramics based on Al₂O₃ orother biocompatible, oxidic or non-oxidic ceramic materials can be used.The source materials for the electrically conductive vias 203 maytypically be dispersions and solutions of metal salts and metalcomplexes provided to a 3D printer in a fluid environment. Thesematerials may include electrically conductive metal material,electrically conductive ceramics, and electrically conductive oxidematerials; for example, platinum, iridium, niobium, tantalum, palladiumand their alloys. For example, for a medical application, a palladiumalloy may contain silver, copper and/or nickel, such as an alloy of 75%palladium and between 3% and 20% tin, aluminium and/or tantalum, and/orother metallic additives such as niobium, tungsten, molybdenum,zirconium and titanium.

The 3D printer deposits these materials layer by layer in a 3D printingprocess, steps 302 and 303, that finally forms the 3D printed electricalfeedthrough arrangement 200. In the initially printed electricalfeedthrough arrangement 200 the source materials of the printed layersstill exist in a fluid environment. So there is interfusion between thesource material layers of the electrical insulator 202 and the one ormore electrically conductive vias 203 so that their respective materialsintermingle and diffuse into each other so that the interfaces betweenthem are not clearly separated or sharply defined. Thus, around each ofthe conductive vias 203 there is a transition interface region 204 thatis a gradient mixture of the ceramic insulator material and themetallized conductive material.

In some cases, the materials used to print the electrically conductivevias 203 may include some of the ceramic insulator materials to achievea predefined resistivity value. Using a mixture of printing materialsfor the electrically conductive vias 203 may also result in betterbonding between the electrical insulator 202 and the one or moreelectrically conductive vias 203. Controlling a mixture of the printingmaterials may also be performed to develop gradients in the transitioninterface region 204 to develop a more conductive region near theelectrically conductive via 203, transitioning to a more electricallyinsulating region near the electrical insulator 202. Such materialgradients would also contribute to better bonding between the electricalinsulator 202 and the one or more electrically conductive vias 203.

The electrically conductive vias 203 and the electrical insulator 202may be formed into any desired geometry that can be realized by theadditive 3D printing process. For example, the electrically conductivevias 203 may be straight, or in a helical shape to form an inductivecoil.

After printing by the 3D printing process, the electrical feedthrougharrangement 200 is then sintered, step 304, to hermetically seal thecombination of the electrical insulator 202 and the one or moreelectrically conductive vias 203. The sintering process avoids formationof a connection by mere compression between the electrical insulator 202and the one or more electrically conductive vias 203. The sintering alsointerfuses the materials in the transition interface region 204 to forma gradual transition and a firm mechanical bond between the electricalinsulator 202 and the conductive via 203.

The electrical insulator 202 also has an insulator outer surface 205made of metallic or ceramic/metallic material that is adapted forhermetic sealing to the metallic outer ferrule 201, steps 305 and 306,for example, by sintering or brazing. This metallized interface of theinsulator outer surface 205 can be created in the 3D printing process,or by some other conventional technique such as sputter metallization oruse of active braze alloys. Besides brazing with pure gold braze,ceramic components can be joined using active brazing alloys, whichavoids the need for ceramic metallisation before brazing. The activecomponents of the brazing alloy promote the wetting of the alloy on theceramic surface. For example, ABA® is a commercially available activebrazing alloy of Morgan Technical Ceramics Wesgo Metals (MTC Wesgo).Formation of intermetallic phases that might result from interaction ofa pre-braze metallisation layer and braze material can be avoided.

Using a 3D printing process to prepare the electrical feedthroughassembly 200 provides many advantages. The 3D printing process isadditive and often applied for rapid prototyping. It provides a flexibleway to create different patterns, sizes and geometries of electricalcontact locations on opposing sides of the electrical feedthroughassembly 200 to facilitate attachment of conductive members, and alsooffers the potential to miniaturize the overall size of the electricalfeedthrough assembly 200. Printing also reduces the number of processesneeded for hermetically sealing the electrical feedthrough assembly 200.The electrical insulator 202 with embedded conductive structures ismanufactured in a single manufacturing process and the number of joiningsteps also is reduced. Moreover, there is no need for handling andassembly of miniature components as is necessary with conventionalbrazing technology.

FIG. 4 shows a cross-sectional view of an electrical feedthroughassembly 400 according to another embodiment which includes one or moreelectrical circuit components embedded within the electrical insulatorin addition to or instead of one or more electrically conductive vias403 with corresponding transition interface regions 404. In suchembodiments, the electrical insulator 402 besides having a metallizedouter surface 405 for hermetic sealing to an outer ferrule (not shownhere) also serves as a substrate for embedded electrical components,like resistors, capacitors, etc., or even entire conductive circuits ofmultiple such elements. So as shown in FIG. 4, an electrical componentC1 may be electrically connected to another electrical component C2 bywire W6. Wire W6 may be made in the same kind of 3D printing process asthe electrically conductive vias of FIG. 2. On the right side of FIG. 4,the electric components C3 and C4 are similarly connected via wire W7and resistor R5. Whereas wire W7 may be made the same way as wire W6,resistor R5 includes more of the electrical insulator material to form ahigher ohmic resistance.

In specific embodiments, such electric components and circuits may belocated on either or both sides of the electrical insulator 402, and/orthey may be embedded within the interior volume of the electricalinsulator 402. Even more complicated circuit structures may be realizedwith the 3D printing process.

Shrinkage of the printing materials during sintering may not be easy tocontrol. And for the 3D printing process, the powder bed and suspensionsneed to be adapted to avoid formation of cracks upon co-sintering. The3D printing process is based on powder-metallurgical techniques whichlead to porous materials after sintering, and that porosity has to becontrolled in order to obtain a hermetically sealed feedthroughassembly. Sintering in combination may need to be performed in areducing atmosphere to avoid oxidation of the metal, but such a reducingatmosphere may affect the insulating material. Sintering in an oxidizingatmosphere is possible for inert electrically conductive materials usedfor the electrically conducting vias or the termination of the vias atthe surface of the electrical feedthrough assembly.

Although various exemplary embodiments of the invention have beendisclosed, it should be apparent to those skilled in the art thatvarious changes and modifications can be made which will achieve some ofthe advantages of the invention without departing from the true scope ofthe invention.

What is claimed is:
 1. A method of producing an electrical feedthroughassembly for an implantable medical device, the method comprising:providing an outer ferrule of metallic material with an outer surfacehermetically sealed to an implantable device housing; producing an innerfeedthrough assembly, the producing including: i. providing ceramicinsulator material in a fluid environment to form an electricalinsulator by a 3D printing process, ii. providing metallized conductivematerial in a fluid environment to form one or more electricallyconductive vias by the 3D printing process, the one or more electricallyconductive vias embedded within and extending through the electricalinsulator, and iii. sintering the ceramic insulator material and themetallized conductive material so as to allow the ceramic insulatormaterial and the metallized conductive material to intermingle anddiffuse into each other to form a transition interface region, aroundeach of the one or more conductive vias, that includes a gradual changein composition and a mechanical bond between the electrical insulatorand the one or more electrically conductive vias, wherein the sinteringis performed in a reducing atmosphere or an oxidizing atmosphere inorder to reduce porosity in the inner feedthrough assembly; locating theinner feedthrough assembly within the outer ferrule; and forming ahermetic seal between the inner feedthrough assembly and the outerferrule.
 2. A method according to claim 1, wherein the hermetic sealbetween the inner feedthrough assembly and the outer ferrule is formedby sintering.
 3. A method according to claim 1, wherein the hermeticseal between the inner feedthrough assembly and the outer ferrule isformed by brazing.
 4. A method according to claim 1, wherein producingthe inner feedthrough assembly includes embedding one or more electricalcircuit components within the electrical insulator.
 5. A methodaccording to claim 1, wherein the ceramic insulator material and the oneor more electrically conductive vias are produced by a 3D printingprocess.
 6. A method according to claim 5, wherein the ceramic insulatormaterial includes ceramic powder particles.
 7. A method according toclaim 1, wherein the implantable medical device is a cochlear implantdevice.